1. Field of the Invention
This invention relates to safety devices for use during maintenance work on live, high voltage electrical transmission lines and more particularly to portable protective air gap devices employed in such circumstances and related methodology for designing such devices for differing line voltage systems.
2. Background of the Invention
Of course, high voltage, electrical transmission systems require routine maintenance to ensure their continuing integrity. A majority of utilities in the country resort to inactivating the line segment under repair resulting in temporary but costly inconveniences to users.
A small number of utilities have sought to develop tools and techniques which will allow for maintenance of these lines while still energized. Standards for such maintenance efforts exist, including the IEEE's Standard 516, “Guide For Maintenance Methods On Energized Power Lines” and OSHA's Regulation, 29 CFR 1910.269.
A major safety concern surrounding such live line work is the presence of overvoltages on the line. Overvoltages are voltage levels that are in excess of the normal power frequency system voltage. They occur for various reasons, the more common of which are lightning, switching operations and line faults.
The principal area of concern, realistically speaking, under standards set for live line maintenance are overvoltages due to switching operations. These very high overvoltages occur when, for example, a line breaker opens a breaker to clear a single-phase fault; or, a line breaker attempts to re-close a line following a momentary single-phase fault clearing operation. Both acts give rise to the overvoltage condition; but the act of reclosing most often gives rise to higher overvoltages.
The clearances to transmission line tower structures are designed to withstand these overvoltages. However the presence of tools and workers at the worksite, and other factors, such as broken insulators, lower the worksite “withstand voltage”, and a breakdown can occur when a sufficiently high, switching surge reaches the worksite. Thus a need for protection exists.
One approach to reducing the magnitude of worksite, overvoltages is to block the re-closure of a circuit breaker—i.e., the breaker is prevented from re-closing following the first trip signal. This will eliminate the source of the very high overvoltages at the worksite. However, this blocking feature cannot be visually confirmed by the line workers, so that its effectiveness is questionable, certainly, at least, in so far as the psychological needs of the line workers. Blocking reclosures also do not guarantee that overvoltages resulting from the sole act of opening the breaker are sufficiently, acceptably low for worksite withstand voltage.
Another approach that is employed to enhance workers' safety in the live line maintenance scenario is to employ what is referred to as a portable protective air gap device (PPAG). This tool seeks to assure positive control at the worksite itself over the maximum voltage that can electrically stress the air gap at a worksite.
With safety as its primary consideration, the assignee hereof has chosen to employ PPAGs as the most direct and visible (to the work crew) method of worksite overvoltage control. The PPAG will spark over at a predetermined voltage that will prevent possible sparkover at the worksite. These tools can allow the worker's minimum approach distance (MAD), determined without the use of the PPAG, to be reduced to a withstand distance coordinated with the sparkover voltage of the PPAG. By reducing the MAD, the use of shorter and lighter tools and better control by the worker in handling tools and small parts is facilitated.
Heretofore the Assignee hereof, PSE&G, has employed a particular design of PPAG in live line work on its 500 kV systems. The tool employed includes a fiberglass insulating rod or “hot stick”, available from A.B. Chance Co. of Centralia, Mo. The tool includes two permanently installed electrodes positioned on the rod to form a horn shaped air gap. One electrode is connected to a live line through an appropriate clamp. The remaining electrode is connected through an appropriate clamp and cable, to the tower structure at ground potential. FIG. 1 depicts the prior art tool of the Assignee herein which was designed for use on 500 kV lines. For that application, the air distance between the rods forming the horn-shaped air gap is 41″. This tool was developed in the 1970's through an extensive trial and error program, which lasted until something was achieved that worked.
As noted above, PPAGs are used to enhance the safety at a worksite where maintenance on live electrical transmission lines is to take place. The function is to limit overvoltage conditions at the worksite on the line under repair. The fact that the tool is set up by the lineman himself according to procedures in which he is trained enhances the psychological advantages of this technique.
In addition to the PPAG depicted as prior art in FIG. 1, alternative approaches have been evaluated. One uses two hollow spheres each positioned at the end of a rod connected respectively to a live line and ground. These spheres are susceptible to denting and other damage. As a result the practical utilization of this technique is minimal. Another approach involves the use of one sphere cooperating with a flat, sheet-like electrode to define the air gap. This suffers from the denting problems associated with spheres and is otherwise unwieldy for practical application.
Because of the somewhat random character of sparkover, the exact values of the spark-over voltage cannot be determined by tests. For practical reasons, therefore, the probability of spark over at a specific line voltage value must be ascertained with consideration given to worksite structures and conditions.
It also became apparent that to extend live line maintenance capability to a majority of the transmission voltage schemes presently employed by utilities, a PPAG device adaptable so as to be employed with a breadth of line voltages is desirable.
At voltages of 345 kV and above, insulated hotstick tools are bulky, heavy, and, because of their length, difficult to control without the use of a shepherd hook or nonconductive rope. A practical, workable answer is required.